The present invention relates to an electrically operated variable force solenoid pressure control valve of the type which may be employed for use in automatic transmission controllers for controllably reducing a fluid inlet pressure to an outlet control pressure.
A known variable force solenoid pressure control valve has utilized a stepped diameter spool. The differential area between the stepped diameters is used to apply a known force proportional to the control or output pressure which known force can more readily be overcome by the solenoid to provide desired actuation of the spool.
Referring to FIG. 4, a known stepped diameter spool valve is shown with the solenoid omitted for the sake of brevity having an operating rod 1 connected to the solenoid armature which moves spool 2 having a large diameter valving land 3 movable in spool bore 4 with a reduced diameter valving land 5 movable in reduced diameter spool bore 6. Land 3 valves flow to exhaust port 7; whereas, land 5 valves flow from inlet port 8 to annular recess 9 which is continuously connected with the control pressure outlet port 13.
However, the manufacture of a valve having a stepped bore and stepped spool requires that extremely fine control of the concentricity between the stepped diameters be maintained. This results in very tight tolerances on the machining of the spool and the bore and therefore increases the cost of the valve resulting in a prohibitively expensive valve construction for high volume mass production motor vehicle automatic transmission usage. Therefore, it has been desired to provide a way or means of utilizing the differential area technique to control the force required to actuate the valve spool against the pressure forces of the hydraulic fluid and yet provide such a valve which is relatively easy to manufacture and which eliminates the need for control of concentricity between different diameters and unusually tight machining tolerances.
Furthermore, it has been desired to reduce the effective pressure transients during the operation of the valve in a hydraulic control system such as the shift control circuit of an automatic transmission in a manner which is cost effective. Heretofore, variable force solenoid valves have employed annular grooves in the valve spool bore to provide flow of fluid for balancing the pressure forces on the spool. However, it has proven costly to machine such grooves in the spool bore; and, therefore it has been desired to eliminate the need for such costly manufacturing operations and yet provide a way to enable the flow of fluid for balancing pressure forces on the spool.